Heretofore, in order to continuously cast a hollow metallic ingot such as aluminum alloy usually having a cross-section in the circular form, a water-cooled metallic core member having the cross-section of the outer peripheral surface so shaped as to define the configuration in cross-section of the inner peripheral surface of the hollow metallic ingot to be continuously cast is placed in position in the inner space of an open ended annular water-cooled metallic mold having the cross-section of the inner peripheral surface so shaped as to define the configuration in cross-section of the outer peripheral surface of the hollow ingot so that an open ended annular casting passage is formed between the mold and the core member. A movable cradle or an annular supporting base member is first located so as to close the lower open end of the casting passage, and, thereafter, molten metal is continuously supplied into the upper open end of the casting passage so as to cause solidification of the molten metal at an appropriate position between the upper and lower ends of the casting passage by forcibly cooling the mold and the core member. After the solidification of the molten metal has commenced, the annular supporting base member is gradually lowered apart from the lower end of the casting passage together with the base member so as to be water-cooled from the inside and the outside of the solidified hollow metal to form the hollow ingot, while the molten metal is continuously supplied into the upper end of the casting passage so as to compensate for the amount of the metal pulled out from the casting passage, thereby maintaining the steady state of the continuous casting operation. The supply of the molten metal in the casting passage for the initial charge and thereafter for supplementing the amount of the solidified metal pulled out from the casting passage is usually effected by a plurality of molten metal supplying devices directly located in spaced relationship from each other in the annular casting passage. Each molten metal supply device is provided with a molten metal level controller consisting of a float and dip tubes with the lower ends extending in the flat bottomed recess in the float so that, when the level of the molten metal descends to lower the float, the lower ends of the tubes are spaced from the flat bottom so as to supply the molten metal through the tubes, whereas, when the level is raised so as to cause abutment of the flat bottom against the lower ends of the tubes, the supply of the molten metal through the tubes is stopped so as to maintain the level constant.
With such an apparatus as described above wherein the inner and outer peripheral surfaces of the hollow ingot to be continuously cast are forcibly cooled by the water-cooled mold and the water-cooled core member, the following difficulties can not be avoided. That is, solidified shells are first formed at the outer peripheral surface and the inner peripheral surface of the molten metal supplied into the casting passage and, thereafter, the solidification of the molten metal proceeds toward the interior thereof between the outer and inner peripheral surfaces, thereby resulting in shrinkage of the metal between the outer and inner solidified shells. Therefore, since the shells are rigid and will not be subjected to shrinkage, cracks tend to occur in the interior of the solidified metal. Further, since the amount of shrinkage at the side of the inner peripheral portion of the solidified metal is greater due to the direct cooling effect given by the water-cooled core member, the solidified metal firmly clamps the core member so that cracks tend to occur in the inner peripheral portion, thereby disturbing smooth continuous casting operation. In order to avoid the above difficulty, the core member is so shaped that the outer peripheral surface is tapered downwardly inwardly so as to facilitate pulling out of the solidified metal downwardly from the casting passage. However, due to the tapered configuration of the core member, occurrence of sweating or cold shut in the inner peripheral surface of the solidified metal can not be avoided depending on the casting condition employed as the solidified metal is pulled out from the casting passage, thereby resulting in unevenness in the inner peripheral surface and making it impossible to obtain a smooth inner peripheral surface. Since a large amount of metal must be removed by scaling operation in order to remove the defects in the inner peripheral portion of the hollow ingot, the yield in production thereof is largely lowered. Thus, the defects in the inner peripheral portion of the hollow ingot afford severe problems to the production of the hollow ingot by continuous casting operation.
Further, since the molten metal supplying devices each having the level controllers are placed directly in the casting passage requiring substantial space therefor, the wall-thickness of the hollow ingot is limited to about 80 mm at the thinnest, thereby making it difficult to produce hollow ingot having a thinner wall-thickness. Further, by the apparatus having the core member with the outer peripheral surface thereof tapered downwardly inwardly, clearance might be formed between the tapered outer peripheral surface of the core member and the solidified shell formed by forcibly cooling in the inner peripheral portion of the solidified metal as the same is pulled out from the casting passage, thereby resulting in danger to cause leakage of the molten metal through the clearance so as to make it impossible to continue the casting.
In order to avoid the above described difficulty, efforts have been made to form the core member integrally as a whole by graphite so as to be able to cast the hollow ingot by suppressing the cooling effect at the inner peripheral portion. The graphite has a great heat capacity and a high thermal conductivity as well as a superior lubricating property. However, even though the core member is formed by the graphite, the formation of the solidified shell in the inner peripheral portion of the molten metal and the danger of leakage of the molten metal through the clearance formed between the tapered outer peripheral surface of the core member and the solidified shell as described above as the solidified metal is pulled out of the casting passage can not be avoided, because the heat capacity of the graphite forming the core member is great thereby resulting in a high initial cooling effect given by the core member. Further, even though the leakage of the molten metal through the clearance as described above be avoided as the solidified metal is pulled out from the casting passage, the great cooling effect of the core member made of graphite remains for a fairly long time, thereby resulting in great heat removing effect given to the molten metal, and the unevenness of the inner peripheral surface described previously can not be avoided so that the yield in casting is greatly deteriorated to prevent desirable continuous casting operation from being carried out. In order to avoid the above difficulty, however, preheating of the core member as well as the provision of heating means in the core member is practically difficult.
Further, since the molten metal supplying devices each having the level controllers are arranged directly in the casting passage formed between the mold and the core member, thin-walled hollow ingot can not be produced as described previously. It has been proposed to support the core member in the dipped state in the molten metal so as to provide a molten metal pool at the upper side of the core member and to arrange the molten metal supplying spout in the above molten metal pool. With such an arrangement, however, the solidification of the molten metal at the surface of the core member, particularly at the upper surface of the core member, is accelerated or expedited due to the great cooling effect of the core member because the rate of supply of the molten metal is rather little because of the hollow configuration of the ingot to be continuously cast. This tendency is rendered to be greater as the wall-thickness of the hollow ingot is made thinner, and as the diameter of the ingot is made larger. As a result, the hollow ingot thus cast can not be pulled out from the casting passage or the core member is forcibly pulled out together with the ingot sticking thereto, thereby tending to make it impossible to continue the casting operation. Further, the temperature of the molten metal tends to descend non-uniformly, thereby resulting in formation of floating crystals within the ingot so that the structure of the finished surface of the ingot after scalping operation is given thereto is made non-uniform resulting in the spotted or streaked finished surface of the product.
In order to avoid the above difficulties caused by the thermal property of the graphite forming the core member described above, it has been proposed to form the casting face on which the solidification of the molten metal is effected by a heat-insulating material. However, such a heat-insulating material has a deteriorated lubricating property in comparison with the graphite, thereby making it difficult to form a superior inner peripheral surface of the ingot. Further, since the mechanical strength of the heat-insulating material is low, it tends to be broken during the casting operation. Particularly, it is easily broken when the cooling water beats against it.
Further, it has also been proposed to provide a molten metal receiving vessel (a so called "Hot Top"), made of a refractory material on the annular water-cooled mold used for defining the outer peripheral surface of the hollow ingot to be cast, and to locate centrally in the inner space of the mold a core member used for defining the inner peripheral surface of the hollow ingot, so that the molten metal supplied to a molten level controller is introduced therefrom into the vessel through a molten metal distributing passage, thereby permitting the molten metal to be continuously supplied from the vessel into the casting passage from between the mold and the core member so as to carry out the continuous casting operation of the hollow ingot. In this case, when a plurality of sets of the above apparatus are provided in order to carry out simultaneously multiple casting operations, each water-cooled mold must be provided with a molten metal receiving vessel while at least a molten metal distributing passages must be provided leading from the level controller to each of the respective vessels, thereby not only making the overall construction of the apparatus complicated but also rendering the temperature of the molten metal supplied to the respective vessels and hence to the respective casting passages to be different from each other and even in such casting passage. Further, provision of a plurality of molten metal distributing passages for achieving the uniform temperature of the molten metal to each of the vessels will make the arrangement, configuration and the number of the distributing passages very difficult. In other words, the workability and the productivity of the hollow ingot by the continuous casting operation are deteriorated, and the production of ingots of a high quality is made difficult. When the temperature is made non-uniform, floating crystals are formed in the ingot as described previously, and the structure of the finished surface of the ingot after cutting out the defects in the surface of the ingot is made non-uniform, resulting in spotted and streaked appearance.